IAEA International Atomic Energy Agency RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY L16.1: Optimization of protection in fluoroscopy

IAEAInternational Atomic Energy Agency

RADIATION PROTECTION INDIAGNOSTIC AND

INTERVENTIONAL RADIOLOGY

L16.1: Optimization of protection in fluoroscopy

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

IAEA 16.1: Optimization of protection in fluoroscopy 2

Introduction

• Subject matter : fluoroscopy equipment and accessories

• Different electronic component contribute to the image formation in fluoroscopy.

• Good knowledge of their respective role and consistent Quality Control policy are the essential tools for an appropriate use of such equipment.


Topics

Example of fluoroscopy systems

Image intensifier component and parameters

Image intensifier and TV system


Overview

• To become familiar with the components of the fluoroscopy system (design, technical parameters that affect the fluoroscopic image quality and Quality Control).


Part 16.1: Optimization of protection in fluoroscopy

Topic 1: Example of fluoroscopy systems



• Used to visualize motion of internal fluid, structures

• Operator controls activation of tube and position over patient

• Early fluoroscopy gave dim image on fluorescent screen

• Modern systems include image intensifier with television screen display and choice of recording devices

Fluoroscopy: a “see-through” operation with motion


• X-ray transmitted trough patient• The photographic plate replaced by fluorescent screen• Screen fluoresces under irradiation and gives a live

image• Older systems— direct viewing of screen• Screen part of an Image Intensifier system• Coupled to a television camera• Radiologist can watch the images “live” on TV-monitor;

images can be recorded• Fluoroscopy often used to observe digestive tract

• Upper GI series, Barium Swallow• Lower GI series Barium Enema

Fluoroscopy


Direct Fluoroscopy: obsolete

In older fluoroscopic examinations radiologist stands behind screen and view the pictureRadiologist receives high exposure; despite protective glass, lead shielding in stand, apron and perhaps goggles

Main source of staff exposure is NOT the patient but direct beam


Older Fluoroscopic Equipment(still in use in some countries)

Staff in DIRECT beam


• AVOID USE OF DIRECT FLUOROSCOPY

• Directive 97/43Euratom Art 8.4.

• In the case of fluoroscopy, examinations without an image intensification or equivalent techniques are not justified and shall therefore be prohibited.

• Direct fluoroscopy will not comply with BSS

• “… performance of diagnostic radiography and fluoroscopy equipment and of nuclear medicine equipment should be assessed on the basis of comparison with the diagnostic reference levels

Direct fluoroscopy


Modern Image Intensifier based fluoroscopy system


Automatic controldisplay brightnessradiation dosefilm exposure

Timer

Display control

Modern fluoroscopic system components


Different fluoroscopy systems

• Remote control systems• Not requiring the presence

of medical specialists inside the X Ray room

• Mobile C-arms• Mostly used in surgical

theatres.


Different fluoroscopy systems

• Interventional radiology systems• Requires specific safety considerations.

In interventional radiology the physician can be near the patient during the procedure.

• Multipurpose fluoroscopy systems • Can be used as a remote control system

or as a system to perform simple interventional procedures



Topic 2: Image Intensifier component and parameters



The image intensifier (I.I.)

+

I.I. Input Screen

I.I.Output Screen

Photocathode

Electrode E1

Electrode E3

Electrode E2

Electrons Path


Image intensifier systems


Image intensifier component

Input screen: conversion of incident X Rays into light photons (CsI)

1 X Ray photon creates 3,000 light photons

Photocathode: conversion of light photons into electrons only 10 to 20% of light photons are converted into

photoelectrons

Electrodes : focusing of electrons onto the output screen electrodes provide the electronic de-magnification

Output screen: conversion of accelerated electrons into light photons


Image intensifier parameters (I)

Conversion coefficient (Gx): the ratio of the output screen brightness to the input screen dose rate—

Gx = cd.m-2Gys-1

Gx depends on :

the applied tube potential

the diameter () of the input screen

I.I. input screen () of 22 cm Gx = 200

I.I. input screen () of 16 cm Gx = 200 x (16/22)2 = 105

I.I. input screen () of 11 cm Gx = 200 x (11/22)2 = 50


Image intensifier parameters (II)

• Brightness Uniformity: the input screen brightness may vary from the center of the image intensifier to the periphery

• Geometrical distortion: all X Ray image intensifiers

exhibit some degree of pincushion distortion. This is

usually caused by a local magnetic field.

Uniformity = (Brightness(c) - Brightness(p)) x 100 / Brightness(c)


Image distortion


Image intensifier parameters (III)

Spatial resolution limit: the value of the highest spatial frequency that can be visually detected it provides a sensitive measure of the state of

focusing of a system it is quoted by manufacturer and usually measured

optically and under fully optimized conditions. This value correlates well with the high frequency limit of the Modulation Transfer Function (MTF)

it can be assessed with a resolution pattern which contains several sets of patterns at various frequency


Line pair gauges


Line pair gauges

GOOD RESOLUTION POOR RESOLUTION


Image intensifier parameters (IV)

Overall image quality - threshold contrast-detail detection

X Ray, electrons and light scatter process in an I.I. can result in a significant loss of contrast of radiological detail. The degree of contrast exhibited by an I.I. is defined by the design of the image tube and coupling optics. Spurious sources of contrast loss are:

accumulation of dust and dirt on the various optical surfaces reduction in the quality of the vacuum aging process (deterioration of phosphor screen)

Sources of noise are: X Ray quantum mottle photon-conversion processes, film granularity, film processing


Image intensifier parameters (V)

• Overall image quality can be assessed using a suitable threshold contrast-detail detectability test object which comprises an array of disc-shaped metal details and gives a range of diameters and X Ray transmission

• Sources of image degradation such as contrast loss, noise and unsharpness limit the number of details that are visible.

• If performance is regularly monitored using this test, any sudden or gradual deterioration in image quality can be detected as a reduction in the number of low contrast or small details.


Overall image quality



Topic 3: Image Intensifier and TV system



Image intensifier - TV system

Output screen image can be transferred to different optical displaying systems: conventional TV

262,5 odd lines and 262,5 even lines generating a full frame of 525 lines (in USA)

625 lines and 25 full frames/s up to 1000 lines (in Europe) interlaced mode is used to prevent flickering

Cine film 35 mm film format: from 25 to 150 images/s

photography roll film,105 mm: max 6 images/s film of 100 mm x 100 mm


VIDICONVIDICON

FILMFILM PMPM REFERENCEREFERENCEkVkV

CONTROLLERCONTROLLER

X Ray TUBEX Ray TUBE kVkV

GENERAL SCHEME OF FLUOROSCOPYGENERAL SCHEME OF FLUOROSCOPY


VIDICON

FILMPM

CONTROLLER

X Ray TUBEkV

CINE MODECINE MODE

I2

Ref.

I3

C1

I1

C2


Type of TV camera

VIDICON TV camera improvement of contrast improvement of signal to noise ratio high image lag

PLUMBICON TV camera (suitable for cardiology) lower image lag (follow up of organ motion) higher quantum noise level

CCD TV camera (digital fluoroscopy) digital fluoroscopy spot films are limited in resolution,

since they depend on the TV camera (no better than about 2 c/mm) for a 1000 line TV system


TV camera and video signal (I)

• The output phosphor of the image intensifier is optically coupled to a television camera system. A pair of lenses focuses the output image onto the input surface of the television camera.

• Often a beam splitting mirror is interposed between the two lenses. The purpose of this mirror is to reflect part of the light produced by the image intensifier onto a 100 mm camera or cine camera.

• Typically, the mirror will reflect 90% of the incident light and transmit 10% onto the television camera.


TV camera and video signal (II)

• Older fluoroscopy equipment will have a television system using a camera tube.

• The camera tube has a glass envelope containing a thin conductive layer coated onto the inside surface of the glass envelope.

• In a PLUMBICON tube, this material is made out of lead oxide, whereas antimony trisulphide is used in a VIDICON tube.


Photoconductive camera tube

Focussing optical lens

Input plate

Steering coils Deviation coilAlignement coil

Accelarator grids

Control grid

Electron beam

Video Signal

Signal electrode Field grid ElectrodeElectron gun

Iris

Photoconductive layer


TV camera and video signal (III)

• The surface of the photoconductor is scanned with an electron beam and the amount of current flowing is related to the amount of light falling on the television camera input surface.

• The scanning electron beam is produced by a heated photocathode. Electrons are emitted into the vacuum and accelerated across the television camera tube by applying a voltage. The electron beam is focussed by a set of focussing coils.


TV camera and video signal (IV)

• This scanning electron beam moves across the surface of the TV camera tube in a series of lines.

• This is achieved by a series of external coils, which are placed on the outside of the camera tube. In a typical television system, the image is formed from a set of 625 lines. On the first pass the set of odd numbered lines are scanned followed by the even numbers. This type of image is called interlaced.

• The purpose of interlacing is to prevent flickering of the television image on the monitor, by increasing the apparent frequency of frames (50 half frames/second).

• In Europe, 25 frames are updated every second.


Different types of scanning

INTERLACED SCANNING

PROGRESSIVESCANNING

12 2

14

4 16

18 6

1

820

13

15

17

10

11

3

21

19

5

7

9

35

1816141210 8 6 4 2

79

11131517

1

625 lines in 40 msi.e. : 25 frames/s


TV camera and video signal (V)

• On most fluoroscopy units, the resolution of the system is governed by the number of lines of the television system.

• Thus, it is possible to improve the high contrast resolution by increasing the number of television lines.

• Some systems have 1,000 or 2,000


TV camera and video signal (VI)

• Many modern fluoroscopy systems used CCD (charge coupled devices) TV cameras.

• The front surface is a mosaic of detectors from which a signal is derived.

• The video signal comprises a set of repetitive synchronizing pulses. In between there is a signal that is produced by the light falling on the camera surface. The synchronizing voltage is used to trigger the TV system to begin sweeping across a raster line.

• Another voltage pulse is used to trigger the system to start rescanning the television field.


Schematic structure of a charged couple device (CCD)


TV camera and video signal (VII)

• A series of electronic circuits move the scanning beams of the TV camera and monitor in synchronism. This is achieved by the synchronizing voltage pulses. The current, which flows down the scanning beam in the TV monitor, is related to that in the TV camera.

• Consequently, the brightness of the image on the video display is proportional to the amount of light falling on the corresponding position on the TV camera.


TV image sampling

SYNCHRO12 µs

LIGHT INTENSITY

SAMPLING

64 µs

VIDEOSIGNAL(1 LINE)

52 µsIMAGE LINE

SINGLE LINE TIME

DIGITIZED SIGNAL

ONE LINE

IMAGE512 x 512PIXELS

WIDTH 512

HEIGHT 512


Digital radiography principle

Clock

MemoryADCI

Iris

t

t

ANALOGUESIGNAL

DIGITALSIGNAL

See more in Lecture L20


Digital Image recording

• In newer fluoroscopic systems film recording is replaced with digital image recording.

• Digital photospots are acquired by recording a digitized video signal and storing it in computer memory.

• Operation— fast, convenient.• Image quality can be enhanced by application

of various image processing techniques, including window-level, frame averaging, and edge enhancement.

• But the spatial resolution of digital photospots is less than that of film images.


• It is possible to adjust the brightness and contrast settings of the TV monitor to improve the quality of the displayed image.

TV camera and video signal (VIII)


Summary

• The main components of the fluoroscopy

imaging chain and their role are explained:

• Image Intensifier

• Associated image TV system


Where to Get More Information

• The Essential Physics of Medical Imaging. JT Bushberg, JA Seibert, EM Leidholdt, JM Boone. Lippincott Williams & Wilkins, Philadelphia, 2011

• The physics of diagnostic imaging, Dowsett et al, Hodder Arnold, 2006

• Interventional Fluoroscopy: Physics, Technology, Safety, S. Balter, Wiley-Liss, 2001

Documents

IAEA International Atomic Energy Agency RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY L16.1: Optimization of protection in fluoroscopy